PROJECT SUMMARY Selectively modulating ?-secretase activity has emerged as a therapeutic strategy for Alzheimer's disease (AD) because it controls the levels of pathogenic A?42 species. We have developed a class of ?-secretase modulators (GSMs), a group of small molecules that specifically modulate ?-secretase processing of APP by binding ?-secretase's PS1 components and thus preferentially lower A?42 levels over A?40, while increasing the levels of shorter A? species. Our GSMs do not affect the cleavage of other ?-secretase substrates, e.g. Notch. Our GSM program evolved several generations of development. Recently, we have developed a novel series of pyridazine class SGSMs which displayed unprecedented level of efficacy in lowering A?42 levels in cell and animal-based studies, with desirable drug-like properties, thus making them outstanding AD clinical candidates. We have reported a lead clinical candidate within this new class, SGSM-15606 (IC50 of A?42 =7nM). This molecule and related analogs have been developed with a plan for a pre-Investigational New Drug (IND) inquiry toward an AD intervention trial. Our GSM program has provided a unique opportunity to characterize the in vivo selectivity, distribution and involvement of ?-secretase in the pathophysiology of AD by molecular imaging. Among the non-invasive imaging techniques, positron emission tomography (PET) is an ideal tool to deliver answers to fundamental questions about ?-secretase in the living human brain. Furthermore, it provides a method to both investigate ?- secretase mechanisms non-invasively and improve drug development. During the past decade, there are several potential disease-modifying therapies for AD advanced to clinical trials. However, the AD therapeutics have a high failure rate for clinical trials partly due to the heterogeneous nature of enrolled patients. In AD trials, patients have been typically recruited under broad diagnostic categories, which is usually related to inconclusive or negative trial outcomes. To address this issue, clinical studies have recently started to apply PET imaging to choose more homogeneous patient groups. For example, the use of PET ligands for A? plaques and neurofibrillary tau tangles have become more routine in clinical trials and enabling better definition of optimal patient groups for clinical trials. In our proposal, we will develop new GSM-based ?-secretase PET imaging probes to serve as a translational tool to help clinical characterization of GSMs by measuring in vivo target occupancy. The results of our studies should facilitate a potential clinical trial of our SGSMs for AD intervention. Additionally, with the availability of our PET probe, the properties of ?-secretase in AD patients could be quantified for the first time, which may ultimately provide a more comprehensive understanding of AD etiology.